One of the major elements present in the slime resulting from copper electrorefining is silver. To recover that silver, the slime is treated by various methods to impure silver anodes. Such anodes are referred to in the art as "Dore" anodes. The composition of a Dore anode greatly varies depending on the source of the slime and of the purity of the original copper anodes, but the silver content is generally from about 80% up to 99%. Dore anodes may also be obtained from lead refining or the treatment of precious metal bearing scrap. Other components or elements of these anodes include copper and precious metals like gold, palladium and platinum.
Dore anodes are refined by electrolysis to produce pure silver metal at the cathode, but this refining also produces anode mud containing gold and other precious metals present in the Dore anode. The silver electrorefining operation is conventionally carried out by using either a Mocbius cell, which is described by Mantell in Electrochemical Engineering, 4.sup.th edition, McGraw Hill Book Company, New York 1960, pp. 166-173; or a Balbach-Thum cell, which is described by de Kay Thompson in Theoretical and Applied Electrochemistry, 3.sup.rd edition, The Macmillan Company, New York, 1939, pp. 257-260. Several considerations will influence the choice of either cell. The Moebius type cell is generally preferred because it requires significantly less floor space, about 1/5 of that of a Balbach-Thum cell, and less energy for a given amount of silver refined. Although the Moebius cell requires more time for removing silver and slime, it needs very little attention during normal operation, as silver crystals building up on the cathodes are scraped mechanically and fall to the bottom of the cell. The Balbach-Thum cell requires frequent manual removal of silver deposited onto the bottom of the cell, which acts as the cathode.
Other significant differences exist between Balbach-Thum cells and Moebius cells, both in the structure and in the physical requirements of the cells, as described in pages 86-87 of Silver: Economics, Metallurgy & Use, (A. Butts & C. D. Coxe), Van Nostrand Company Inc. In a Moebius cell, the anodes and cathodes are suspended in an alternate manner in the cell. The anodes are only partially submerged in the electrolyte which results in a substantial portion of the impure anode being left undissolved ("scrap") at the end of an electrorefining cycle, typically lasting from 24 to 48 hours. The weight of the remaining anode scrap can amount to as high as 30% of the Dore anode originally loaded in the refining cell, and therefore it must be remelted, recast and reelectrolysed, thus increasing the overall costs for obtaining pure silver. On the other hand, in Balbach-Thum cells, the cathode is at the bottom of the cell, and the anodes are deposited at the bottom basket, parallel to the cathode, the bottom of the basket being lined with a cloth to collect the gold mud. Although complete dissolution of the silver anodes appears to occur in Balbach-Thum cells, there are significant manipulations of partially corroded silver anodes for the following reason. As stated above, the anodes are deposited onto the cloth in the basket. Since the anode contains important amounts of impurities, these impurities remain in the basket as anodes dissolve to leave a residue that is referred to in the art as gold mud. After a certain time, the dissolution of silver is impaired by the increasing amount of gold mud in the cloth, and accordingly, gold mud, together with the corroded anodes present therein, must be removed from the basket and the undissolved portion of the anodes must be washed before being returned in the cell.
Both types of cells have in common that the handling of partially corroded anodes and the recovery of gold mud are time-consuming operations, and therefore, any improvement in that respect will result in lower costs for silver refiners.
U.S. Pat. No. 5,100,528 (Claessens et al.) discloses a continuous silver refining cell wherein silver anodes are deposited in a titanium anode basket that is subsequently immersed in a tank containing tile electrolyte. Another silver electrorefining cell has been developed to reduce as much as possible anode scrap, as described by Imazawa et al in "Continuous Silver Electrorefining Operation", Metallurgical Review of the MMIJ, 1984, Vol. 1, No. 1, pp. 150-159. In this cell, tile basket is also made of conductive titanium material to insure contact of the impure silver anode with the positive terminal of the continuous current electrical power source. This cell, as well as the cell described in U.S. Pat. No. 5,100,528, is very complex as it allows for the simultaneous continuous withdrawal of the silver crystals deposited at the cathodes. A further drawback is that they are expensive to build and may be difficult to operate.
The use of conductive baskets is also well known in the plating industry, where replenishment of ions of a metal to be plated is assured by using soluble anodes made of the same metal. In this case, solid anodes may be suspended from the top of the cell, or smaller pieces of the same anode material can be loaded in a partially submerged basket made of inert conductive material. Titanium is conventionally used as material of construction for these baskets. A disadvantage of the use of such conductive baskets in Moebius cells is that some energy is lost at the surface of the basket by the degradation reaction of H.sub.2 O. In addition to the undesirable consumption of energy, this reaction produces O.sub.2 and H.sup.+ ions, the latter increasing the acidity of the electrolyte and impairing the efficiency of the process, since metals like palladium and platinum will dissolve in an electrolyte having a lower pH, thus significantly contaminating the silver.
U.S. Pat. No. 4,692,222 describes the use of a basket made of electrically conductive material substantially inactive to the electrical process, to contain pieces of copper used as replenishment of copper ions in a plating cell. As an alternative, the electrically conductive material may be replaced with plastic, provided that the plastic baskets contain some means of making electrical contact to the pieces of copper therein, such as by way of a conductive rod extending down into the basket. In this instance, because of the presence of the electrical contact in the electrolyte through the conductive rod, the degradation reaction of water will take place, and the acidity of the electrolyte will increase.
U.S. Pat. No. 4,207,153 is concerned with an electrorefining cell that consists of bipolar electrodes having the anode side made of a basket constructed with an acid resistant metal in which fine cemented copper is added in a slurry form. Again in this case, the material of construction of the anode baskets is a metal, such as stainless steel or titanium.
In view of the above, there is therefore a great need to improve the electroefining of silver, particularly in Moebius cells. For example, it would be desirable to develop a method combining the advantages of both Moebius and Balbach-Thum cells, namely allowing the complete dissolution of silver anodes that would be fed in a continuous manner in the electrolyte while eliminating any silver anode residue from the gold mud produced therefrom, thus preventing the manipulation of partially corroded anodes. With such a method, there would no longer be a need to recycle anode scrap by melting and casting, resulting in significant savings in silver production. Further, as mentioned above, the floor space required for a Moebius cell is significantly smaller than that of a Balbach-Thum cell.